1. Field of the Invention
The present invention relates to a superconducting tunable filter, particularly, to a superconducting tunable filter whose center frequency and bandwidth are both adjustable.
2. Description of the Related Art
In recent years and continuing, along with transition to high speed, large capacity data communication, such as the next generation mobile communication system, and a wideband wireless access system, effective utilization of frequency resources becomes indispensable. In order to obtain the best communication access, a communication apparatus supporting plural frequency bands is desirable. A leading candidate for solving the frequency interference problem is high-Q superconducting filter technology, which has low loss, good frequency cutoff characteristics, and a tunable function.
FIG. 1A is a diagram illustrating a micro-strip superconducting resonator filter pattern of the related art.
As shown in FIG. 1A, a micro-strip superconducting resonator filter pattern 112 is formed from superconducting micro-strip lines, and electromagnetic fields are coupled between plural resonators and between resonators and filters, thereby forming a superconducting band-pass filter.
The center frequency or bandwidth, cutoff characteristics, and out-of-band suppression characteristics of the filter are determined by the resonating frequencies fo and resonator coupling coefficients k of the resonators, and an external Q-value. Hence, if the resonating frequencies fo and resonator coupling coefficients k are variable, the filter becomes a tunable filter. For this purpose, from the point of view of material properties, it is sufficient to make at least one of the effective relative permittivity ∈eff and the effective relative permeability μeff variable; alternatively, from the point of view of circuitry, it is sufficient to make at least one of capacitance C and inductance L variable.
However, in order to maintain the performance of a high Q-value filter, it is necessary to avoid increase of the loss caused by the tunable mechanism. In the related art, a tunable filter can be realized by three methods, namely, electric field control, magnetic field control, and mechanical control. Among the three methods, mechanical control can provides the largest tunability, and hence it is anticipated to be an effective method to maintain low loss.
FIG. 1B is a view illustrating a superconducting tunable filter in the related art.
As shown in FIG. 1B, a micro-strip superconducting resonator filter pattern 112 is formed on a dielectric substrate 111, and a dielectric plate or a magnetic plate 120, which has low loss, is arranged on the micro-strip superconducting resonator filter pattern 112. An actuator 121, such as a piezoelectric element, is used to change the distance between the micro-strip superconducting resonator filter pattern 112 and the dielectric (or magnetic) plate 120. Thereby, the effective relative permittivity ∈eff or the effective relative permeability μeff is changed, and thereby, obtaining a tunable filter.
For example, Japanese Patent Gazette No. 3535469 discloses such a technique.
However, in the related art, since the entire upper surface of the micro-strip superconducting resonator filter pattern 112 is covered by the dielectric plate 120, the bandwidth ends up being changed when the variable range of the center frequency increases, and this limits the tunable range. Although it is possible to adjust the in-band characteristics (such as, the bandwidth) of the filter by inserting a dielectric rod or a magnetic rod from the upper side, in this case it is difficult to make the center frequency variable.
Japanese Laid-Open Patent Application No. 2002-57506 discloses a technique for adjusting the in-band characteristics by inserting a dielectric rod or a magnetic rod from the upper side.
As described above, in the related art, although it is possible to make one of the center frequency and the bandwidth tunable, it is difficult to separately change both of the center frequency and the bandwidth.